U.S. patent application number 10/293591 was filed with the patent office on 2004-05-13 for advanced mask cleaning and handling.
This patent application is currently assigned to Applid Materials Israel Ltd. Invention is credited to Yogev, David.
Application Number | 20040090605 10/293591 |
Document ID | / |
Family ID | 32229680 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040090605 |
Kind Code |
A1 |
Yogev, David |
May 13, 2004 |
Advanced mask cleaning and handling
Abstract
Apparatus for semiconductor device fabrication, includes at
least one lithography station, which is adapted to project a
pattern of radiation from a mask onto a semiconductor wafer. A mask
cleaning station is adapted to receive the mask from the at least
one lithography station, to clean the mask so as to remove a
contaminant therefrom, and so that the cleaned mask may be returned
to the at least one lithography station. A robot is adapted to
convey the mask between the at least one lithography station and
the mask cleaning station. An enclosure contains the at least one
lithography station, the mask cleaning station and the robot, so
that the mask is conveyed between the at least one lithography
station and the mask cleaning station without human contact and
without exposure to ambient air.
Inventors: |
Yogev, David; (Nesher,
IL) |
Correspondence
Address: |
PATENT COUNSEL
APPLIED MATERIALS,INC.
Legal Affairs Department
P.O.BOX 450A
Santa Clara
CA
95052
US
|
Assignee: |
Applid Materials Israel Ltd
|
Family ID: |
32229680 |
Appl. No.: |
10/293591 |
Filed: |
November 12, 2002 |
Current U.S.
Class: |
355/30 ;
250/559.4; 250/559.41; 355/53; 356/237.2 |
Current CPC
Class: |
H01L 21/67225 20130101;
H01L 21/67017 20130101; G03F 1/82 20130101 |
Class at
Publication: |
355/030 ;
355/053; 250/559.4; 250/559.41; 356/237.2 |
International
Class: |
G03B 027/52 |
Claims
1. Apparatus for semiconductor device fabrication, comprising: at
least one lithography station, which is adapted to project a
pattern of radiation from a mask onto a semiconductor wafer; a mask
cleaning station, which is adapted to receive the mask from the at
least one lithography station, to clean the mask so as to remove a
contaminant therefrom, so that the cleaned mask may be transferred
to the at least one lithography station; a robot, which is adapted
to convey the mask between the at least one lithography station and
the mask cleaning station; and an enclosure, containing the at
least one lithography station, the mask cleaning station and the
robot, so that the mask is conveyed between the at least one
lithography station and the mask cleaning station without human
contact and without exposure to ambient air.
2. Apparatus according to claim 1, and comprising a mask storage
station, contained at least partly within the enclosure, wherein
the mask storage station is adapted to store the mask, and wherein
the robot is adapted to convey the mask between the mask storage
station and the cleaning and lithography stations.
3. Apparatus according to claim 1, wherein the at least one
lithography station comprises a radiation source for generating the
radiation that is projected onto the semiconductor wafer, and
wherein the radiation has a wavelength that is less than 160
nm.
4. Apparatus according to claim 1, wherein the at least one
lithography station is adapted to project the pattern of radiation
from the mask in the absence of a pellicle covering the mask.
5. Apparatus according to claim 1, wherein the at least one
lithography station comprises a plurality of exposure tools,
commonly contained within the enclosure and served by the mask
cleaning station.
6. Apparatus according to claim 1, and comprising an inspection
station, which is adapted to determine position coordinates of the
contaminant on the mask, and to convey the coordinates to the
cleaning station, which is adapted to clean the mask locally at a
location indicated by the coordinates.
7. Apparatus according to claim 6, wherein the inspection station
is contained within the enclosure.
8. Apparatus according to claim 6, wherein the cleaning station
comprises a vacuum source, which is adapted to apply suction at the
location indicated by the coordinates.
9. Apparatus according to claim 8, wherein the cleaning station
further comprises an inlet port, and the cleaning station is
adapted to inject a fluid medium through the inlet port so that the
fluid medium is deposited on the mask at the location indicated by
the coordinates, prior to applying the suction thereto.
10. Apparatus according to claim 6, wherein the cleaning station
comprises an inlet port, and the cleaning station is adapted to
inject a pressurized cleaning medium through the inlet port so that
the cleaning medium impinges on the mask at the location indicated
by the coordinates.
11. Apparatus according to claim 1, wherein the cleaning station
comprises a radiation source, which is adapted to generate a beam
of electromagnetic energy, and wherein the cleaning station is
adapted to controllably direct the beam of electromagnetic energy
toward a location of the contaminant on the mask, causing the
contaminant to be dislodged from the mask substantially without
damage to the surface itself.
12. Apparatus according to claim 11, wherein the cleaning station
comprises a gas inlet, and the cleaning station is adapted to
inject an energy transfer medium through the gas inlet so that the
medium is deposited on the mask at the location of the
contaminant.
13. Apparatus according to claim 12, wherein the medium absorbs at
least a portion of the electromagnetic energy incident on the mask,
thereby causing local evaporation of the medium, which dislodges
the contaminant.
14. Apparatus according to claim 13, wherein the electromagnetic
energy comprises ultraviolet laser energy.
15. Apparatus according to claim 13, wherein the energy transfer
medium comprises a carrier gas with a condensable vapor.
16. Apparatus according to claim 15, wherein the condensable vapor
is water.
17. Apparatus according to claim 11, wherein the cleaning station
is adapted to receive input position coordinates of the location of
the contaminant on the mask, and to direct the medium and the beam
so that the medium and beam are incident on the mask at the
location indicated by the position coordinates.
18. Apparatus according to claim 17, and comprising an inspection
station, which is adapted to determine the input position
coordinates and to convey the coordinates to the cleaning
station.
19. A method for semiconductor device fabrication, comprising the
steps of: enclosing at least one lithography station and a mask
cleaning station in an enclosure, so that a mask may be conveyed
between the at least one lithography station and the mask cleaning
station without human contact and without exposure to ambient air;
cleaning the mask in the mask cleaning station so as to remove a
contaminant therefrom; conveying the mask within the enclosure from
the mask cleaning station to the at least one lithography station;
and projecting a pattern of radiation from the mask onto a
semiconductor wafer in the at least one lithography station.
20. A method according to claim 19, wherein projecting the pattern
of radiation comprises generating the radiation at a wavelength
that is less than 160 nm.
21. A method according to claim 19, wherein projecting the pattern
of radiation comprises projecting the pattern of radiation from the
mask in the absence of a pellicle covering the mask.
22. A method according to claim 19, and comprising storing the mask
in a mask storage station, which is at least partially contained in
the enclosure.
23. A method according to claim 22, wherein conveying the mask
comprises transferring the mask between the at least one
lithography station, the mask cleaning station, and the mask
storage station.
24. A method according to claim 19, wherein cleaning the mask
comprises determining position coordinates of the contaminant on
the mask, and cleaning the mask locally at a location indicated by
the coordinates.
25. A method according to claim 24, wherein determining the
position coordinates comprises enclosing an inspection station
within the enclosure, and inspecting the mask using the inspection
station.
26. A method according to claim 24, wherein cleaning the mask
locally comprises applying suction at the location indicated by the
coordinates.
27. Apparatus according to claim 26, wherein cleaning the mask
locally further comprises applying a fluid medium to the mask at
the location indicated by the coordinates, prior to applying the
suction thereto.
28. Apparatus according to claim 24, wherein cleaning the mask
locally comprises applying a pressurized cleaning medium to the
mask at the location indicated by the coordinates.
29. A method according to claim 19, wherein cleaning the mask
comprises controllably directing a beam of electromagnetic energy
toward a location of the contaminant on the mask, so as to cause
the contaminant to be dislodged from the mask substantially without
damage to the mask itself.
30. A method according to claim 29, wherein cleaning the mask
further comprises controllably applying an energy transfer medium
at the location of the contaminant on the surface, wherein the beam
of electromagnetic energy causes local evaporation of the medium,
thereby dislodging the contaminant.
31. A method according to claim 30, wherein the electromagnetic
energy comprises ultraviolet laser energy.
32. A method according to claim 30, wherein controllably directing
the electromagnetic energy comprises receiving input position
coordinates of the location of the contaminant on the mask, and
directing the beam so that the beam is incident on the mask at the
location indicated by the position coordinates.
Description
FIELD OF INVENTION
[0001] The present invention relates generally to semiconductor
manufacturing processes, and specifically to methods and apparatus
for cleaning and handling of lithographic masks used in producing
semiconductor devices.
BACKGROUND OF INVENTION
[0002] As the trend continues to reduce the size of semiconductor
devices, optical lithography using conventional transmission masks,
such as chrome on glass (COG) or phase shift (PSM) masks, will no
longer suffice as a viable technique for printing advanced devices
on semiconductor wafers. Transmission lithography has been extended
to ever shorter wavelengths, down to 157 nm in the far ultraviolet
(UV), in order to reduce the size of device features. However, the
still shorter wavelengths necessary for printing even smaller
device structures are readily absorbed in transmission materials.
Alternative technological candidates to replace optical lithography
include: electron projection lithography (EPL) and an
all-reflective technology called extreme ultra-violet lithography
(EUVL).
[0003] Virtually all masks used in production today employ a
pellicle to protect the mask surface from particulate
contamination. The pellicle is a relatively inexpensive, thin,
transparent, flexible sheet, which is stretched above and not
touching the surface of the mask. Pellicles provide a functional
and economic solution to particulate contamination by mechanically
separating particles from the mask surface. The mask is transported
and used for lithographic exposure with the pellicle in place. When
a mask is used for exposure, with the pellicle in position above
the mask, only the details of the mask's focal plane itself are
printed. Particulate material located on the pellicle surface is
maintained outside of the focal plane of projection. As a result,
particulate material is not printed. When the pellicle eventually
becomes damaged or too dirty to use, the mask is removed to a
workshop, and the pellicle is replaced.
[0004] A suitable pellicle material and structure have yet to be
defined for 157 nm technology. The options to date include using
either no pellicle or a very expensive hard pellicle. An
inexpensive soft pellicle that is capable of withstanding multiple
exposures to 157 nm light has yet to be developed. It appears,
therefore, that masks for lithography at 157 nm and for shorter
wavelengths must be used without the protection of a pellicle. If a
no-pellicle option is chosen, the masks must be cleaned frequently,
and the cleaning technique must be suitable for multiple cleaning
cycles without inducing any significant damage to sensitive mask
films. Most contaminants absorb radiation at short wavelengths, and
it is therefore imperative that the mask surface be completely free
of any contamination that may absorb radiation.
[0005] Not only must mask particle contamination removal efficiency
be increased, but the minimum particle size to be removed must also
decrease. For example, in EUV lithography, masks must be cleaned to
remove particles as small as 70 nm, since particles of this size
are already printable at EUV lithography wavelengths. Conventional
cleaning technologies such as sulfuric-peroxide mixture (SPM) and
standard cleans (SC-1 and SC-2) do not fulfill all of the
previously mentioned contamination removal criteria. If these
conventional cleaning procedures must be applied to the mask
repeatedly (due to the absence of a mask pellicle), they are likely
to cause rapid degradation of delicate mask film layers.
[0006] Various methods are known in the art for stripping and
cleaning foreign matter from the surfaces of semiconductor wafers
and masks, while avoiding damage to the surface itself. For
example, U.S. Pat. No. 4,980,536, whose disclosure is incorporated
herein by reference, describes a method and apparatus for removal
of particles from solid-state surfaces by laser bombardment. U.S.
Pat. Nos. 5,099,557 and 5,024,968, whose disclosures are also
incorporated herein by reference, describe methods and apparatus
for removing surface contaminants from a substrate by high-energy
irradiation. The substrate is irradiated by a laser with sufficient
energy to release the particles, while an inert gas flows across
the wafer surface to carry away the released particles.
[0007] U.S. Pat. No. 4,987,286, whose disclosure is likewise
incorporated herein by reference, describes a method and apparatus
for removing minute particles (as small as submicron) from a
surface to which they are adhered. An energy transfer medium,
typically a fluid, is interposed between each particle to be
removed and the surface. The medium is irradiated with laser energy
and absorbs sufficient energy to cause explosive evaporation,
thereby dislodging the particles.
[0008] Various methods are known in the art for discriminating and
localizing defects on substrates. U.S. Pat. Nos. 5,264,912 and
4,628,531, whose disclosures are incorporated herein by reference
are examples. Foreign particles are one type of defects that can be
detected using these methods.
[0009] U.S. Pat. No. 5,023,424, whose disclosure is incorporated
herein by reference, describes a method and apparatus using
laser-induced shock waves to dislodge particles from a wafer
surface. A particle detector is used to locate the positions of
particles on the wafer surface. A laser beam is then focused at a
point above the wafer surface near the position of each of the
particles, in order to produce gas-borne shock waves with peak
pressure gradients sufficient to dislodge and remove the particles.
It is noted that the particles must be dislodged by the shock wave,
rather than vaporized due to absorption of the laser radiation.
U.S. Pat. No. 5,023,424 further notes that immersion of the surface
in a liquid (as in the above-mentioned U.S. Pat. No. 4,987,286, for
example) is unsuitable for use in removing small numbers of
microscopic particles.
SUMMARY OF INVENTION
[0010] It is an object of some aspects of the present invention to
provide improved methods and apparatus for removal of microscopic
particles from lithographic masks used in semiconductor device
production. In the context of the present patent application and in
the claims, the word "particle" is used broadly to refer to any
contaminant or other foreign substance that must be removed from
the mask surface.
[0011] In embodiments of the present invention, a lithography tool,
for use in producing semiconductor devices, comprises one or more
lithography stations, together with a mask cleaning station. The
lithography and mask cleaning stations are contained in a common
enclosure, and a robot is preferably used to transfer the masks
between the cleaning and lithography stations in order to isolate
the mask and the stations from ambient air and from human contact.
This arrangement is particularly advantageous in dealing with masks
without pellicles, since it allows particles to be removed
frequently from the masks, in the production environment, without
removing the masks to a separate mask shop. This arrangement
facilitates the higher level of mask cleanliness that is required
for far UV and EUV lithography.
[0012] Preferably, the lithography tool also comprises an
inspection station, which checks each mask before or after use to
verify that the mask is still clean and, if not, to determine the
locations of any contaminant particles on the mask. If the
inspection station finds the mask to be contaminated, the robot
passes the mask to the cleaning station. Based on coordinates of
the particles determined by the inspection station, the cleaning
station applies a local cleaning process to remove the particles.
Preferably, the local cleaning process involves wetting the
particle location with a suitable fluid, and then irradiating the
location with laser radiation, most preferably UV laser radiation.
This cleaning approach gives optimal removal of contaminant
particles, without affecting in any way the remainder of the
mask.
[0013] Alternatively, various other local cleaning methods may be
used in conjunction with the inspection station. Examples of such
methods include localized plasma application; local application of
pressurized gas or vacuum; and local application of carbon dioxide
dry ice (or "snow cleaning"). In addition, chemical cleaners in
liquid and/or vapor state may be locally dispensed at the particle
coordinates. In any case, when local cleaning is used, degradation
of the mask due to frequent cleaning is minimized, and the useful
life of the mask is thus lengthened.
[0014] There is therefore provided, in accordance with an
embodiment of the present invention, apparatus for semiconductor
device fabrication, including:
[0015] at least one lithography station, which is adapted to
project a pattern of radiation from a mask onto a semiconductor
wafer;
[0016] a mask cleaning station, which is adapted to receive the
mask from the at least one lithography station, to clean the mask
so as to remove a contaminant therefrom, so that the cleaned mask
may be transferred to the at least one lithography station;
[0017] a robot, which is adapted to convey the mask between the at
least one lithography station and the mask cleaning station;
and
[0018] an enclosure, containing the at least one lithography
station, the mask cleaning station and the robot, so that the mask
is conveyed between the at least one lithography station and the
mask cleaning station without human contact and without exposure to
ambient air.
[0019] Preferably, the apparatus includes a mask storage station,
contained at least partly within the enclosure, and the mask
storage station is adapted to store the mask, and the robot is
adapted to convey the mask between the mask storage station and the
cleaning and lithography stations. Further preferably, the at least
one lithography station includes a radiation source for generating
the radiation that is projected onto the semiconductor wafer, and
the radiation has a wavelength that is less than 160 nm.
[0020] In one embodiment, the at least one lithography station is
adapted to project the pattern of radiation from the mask in the
absence of a pellicle covering the mask. The at least one
lithography station may include a plurality of exposure tools,
commonly contained within the enclosure and served by the mask
cleaning station.
[0021] Preferably, the apparatus includes an inspection station,
which is adapted to determine position coordinates of the
contaminant on the mask, and to convey the coordinates to the
cleaning station, which is adapted to clean the mask locally at a
location indicated by the coordinates. Most preferably, the
inspection station is contained within the enclosure. In one
embodiment, the cleaning station includes a vacuum source, which is
adapted to apply suction at the location indicated by the
coordinates. The cleaning station may additionally include an inlet
port, and may be adapted to inject a fluid medium through the inlet
port so that the fluid medium is deposited on the mask at the
location indicated by the coordinates, prior to applying the
suction thereto.
[0022] In another embodiment, the cleaning station includes an
inlet port, and is adapted to inject a pressurized cleaning medium
through the inlet port so that the cleaning medium impinges on the
mask at the location indicated by the coordinates.
[0023] Preferably, the cleaning station includes a radiation
source, which is adapted to generate a beam of electromagnetic
energy, and the cleaning station is adapted to controllably direct
the beam of electromagnetic energy toward a location of the
contaminant on the mask, causing the contaminant to be dislodged
from the mask substantially without damage to the surface itself.
Preferably, the cleaning station includes a gas inlet, and the
cleaning station is adapted to inject an energy transfer medium
through the gas inlet so that the medium is deposited on the mask
at the location of the contaminant. Most preferably, the medium
absorbs at least a portion of the electromagnetic energy incident
on the mask, causing local evaporation of the medium, which
dislodges the contaminant. Preferably, the electromagnetic energy
includes ultraviolet laser energy. Further preferably, the energy
transfer medium includes a carrier gas with a condensable vapor.
Typically, the condensable vapor is water.
[0024] Further preferably, the cleaning station is adapted to
receive input position coordinates of the location of the
contaminant on the mask, and to direct the medium and the beam so
that the medium and beam are incident on the mask at the location
indicated by the position coordinates. Most preferably, the
cleaning station further includes an inspection station, which is
adapted to determine the input position coordinates and to convey
the coordinates to the cleaning station.
[0025] There is also provided, in accordance with an embodiment of
the present invention, a method for semiconductor device
fabrication, including the steps of:
[0026] enclosing at least one lithography station and a mask
cleaning station in an enclosure, so that a mask may be conveyed
between the at least one lithography station and the mask cleaning
station without human contact and without exposure to ambient
air;
[0027] cleaning the mask in the mask cleaning station so as to
remove a contaminant therefrom;
[0028] conveying the mask within the enclosure from the mask
cleaning station to the at least one lithography station; and
[0029] projecting a pattern of radiation from the mask onto a
semiconductor wafer in the at least one lithography station.
[0030] Preferably, projecting the pattern of radiation includes
generating the radiation at a wavelength that is less than 160 nm.
Further preferably, projecting the pattern of radiation includes
projecting the pattern of radiation from the mask in the absence of
a pellicle covering the mask.
[0031] Preferably, the method includes storing the mask in a mask
storage station, which is at least partially contained in the
enclosure, and conveying the mask includes transferring the mask
between the at least one lithography station, the mask cleaning
station, and the mask storage station. Further preferably, cleaning
the mask includes determining position coordinates of the
contaminant on the mask, and cleaning the mask locally at a
location indicated by the coordinates. Most preferably, determining
the position coordinates includes enclosing an inspection station
within the enclosure, and inspecting the mask using the inspection
station.
[0032] Preferably, cleaning the mask includes controllably
directing a beam of electromagnetic energy toward a location of the
contaminant on the mask, so as to cause the contaminant to be
dislodged from the mask substantially without damage to the mask
itself. Further preferably, cleaning the mask also includes
controllably applying an energy transfer medium at the location of
the contaminant on the surface, wherein the beam of electromagnetic
energy causes local evaporation of the medium, thereby dislodging
the contaminant.
[0033] Preferably, the electromagnetic energy includes ultraviolet
laser energy.
[0034] Preferably, controllably directing the electromagnetic
energy includes receiving input position coordinates of the
location of the contaminant on the mask, and directing the beam so
that the beam is incident on the mask at the location indicated by
the position coordinates.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] The present invention will be more fully understood from the
following detailed description of the embodiments thereof, taken
together with the drawings in which:
[0036] FIG. 1 is a schematic illustration of a system for removal
of particles from the surface of a lithographic mask, in accordance
with an embodiment of the present invention;
[0037] FIG. 2 is a schematic, sectional illustration of a
lithographic mask in a particle removal chamber, in accordance with
an embodiment of the present invention;
[0038] FIG. 3 is a schematic top view of a mask on which
contaminant particles have been deposited;
[0039] FIG. 4 is a schematic top view of the mask of FIG. 3
following removal of the contaminant particles using the system of
FIG. 2;
[0040] FIG. 5 is a schematic side view of a system for removal of
particles from the surface of a lithographic mask, in accordance
with another embodiment of the present invention; and
[0041] FIG. 6 is a simplified block diagram showing a lithography
tool with mask handling and particle removal capabilities, in
accordance with an embodiment of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0042] FIG. 1 is a schematic illustration of a system 20 for
removal of particles from the surface of a lithographic mask 26, in
accordance with an embodiment of the present invention. System 20
is similar in certain aspects to cleaning systems described in U.S.
patent application Ser. No. 09/869,058 and in PCT patent
application PCT/IL99/00701, which are assigned to the assignee of
the present patent application and are incorporated herein by
reference.
[0043] System 20 comprises two stations: an inspection station 22
and a particle removal station 24. Typically, stations 22 and 24
are separate entities, as shown in the figure. Inspection station
22 determines the coordinates of contaminant particles 28 on the
surface of the mask. The coordinates are passed to a processor 34,
which stores the coordinates and transforms them to a coordinate
frame of particle removal station 24. Processor 34 has additional
functions, as further described with reference to FIG. 2, below.
Mask 26 is then transferred to particle removal station 24, where
the coordinates are used to direct the removal of the particles
from the surface. Alternatively, stations 22 and 24 are constructed
as a single, integral unit, which both determines the particle
coordinates and removes the particles accordingly, without the need
to transfer the mask from one entity to the next.
[0044] Inspection station 22 may comprise any suitable automated
inspection system known in the art, such as the defect detection
systems mentioned in the Background of the Invention. For example,
the Applied Materials "Compass" or KLA-Tencor "Surfscan" systems
may be used for this purpose. Typically, a laser 30 irradiates the
surface of mask 26, and a detector 32 senses irregularities in the
radiation reflected from the surface. Alternatively, other
inspection methods, such as optical microscopy or scanning electron
microscopy (SEM), may be employed. The irregularities are analyzed
to determine the coordinates of particles 28, and possibly of other
surface defects, as well. Preferably, station 22 is capable of
distinguishing between irregularities due to particles and due to
other causes. Alternatively, if station 22 does not make the
distinction between particles and other defects, station 24 may
attempt (without success) to treat inspected locations of defects
that are not removable particles. Aside from reducing the
throughput of system 20, it is not likely that any harm will result
to mask 26 due to the processing of these non-particle defects by
station 24.
[0045] Particle removal station 24 comprises a laser 36, which
directs an intense beam of energy at the surface of mask 26. In
some embodiments of the present invention, the laser comprises an
excimer laser, such as a Lambda Physik LPX315 IMC laser, which
emits ultraviolet radiation. Alternatively, other laser types and
wavelengths, such as infrared or visible lasers, may be used. The
mask is contained in a chamber 38, which is described in detail
hereinbelow. The laser energy is absorbed at the mask surface,
causing particles 28 to be dislodged from the surface substantially
without damage to the surface itself. Typically, absorption of the
laser energy, by the particles and/or by the mask, causes the
particles to be ablated or otherwise dislodged from the surface, as
described, for example, in the above-mentioned U.S. Pat. No.
5,114,834 or in PCT patent application PCT/IL96/00141, which is
also incorporated herein by reference. Additionally or
alternatively, as described in the above-mentioned U.S. Pat. No.
4,987,286, an energy transfer medium is applied to the surface. The
laser energy, absorbed by the medium and/or by the mask, causes
explosive evaporation, thereby dislodging the particles. Further
alternatively, station 24 may use any other suitable method of
localized particle removal that is known in the art.
[0046] Reference is now made to FIG. 2, which schematically
illustrates details of chamber 38, in accordance with an embodiment
of the present invention. FIG. 2 is a sectional side view. Chamber
38 comprises a rotating chuck 64, on which mask 26 rests securely
(typically by vacuum suction, as is known in the art). The laser
beam irradiates the mask surface at points where inspection station
22 has detected particles. To remove any one of particles 28, chuck
64 is rotated so that the particle is located under the laser beam,
which is fired at the surface in the region of the particle. Radial
scanning of the laser beam may be accomplished either by angular
deflection of the beam, using any sort of suitable optical scanner,
or by translating an optical beam-handling assembly (or even the
entire laser) over the mask in a radial direction. These and other
suitable methods of scanning will be apparent to those skilled in
the art.
[0047] Preferably, a process gas mixed with a vapor is introduced
through a process gas port 56. The vapor condenses to form a liquid
film on the surface. Most preferably, the process gas comprises a
gas or a combination of gases having inert properties, such as
nitrogen, and a vapor such as water vapor. In this case, laser
irradiation causes explosive local evaporation of the liquid,
driving the particles off the mask surface.
[0048] The gas from region 60 is preferably exhausted through one
or more gas exhaust ports 58. As these ports are immediately
adjacent to region 60, the particles removed from the mask surface
will generally be swept immediately out of the region and away from
the mask surface. By minimizing the distance that released
particles must travel over the mask surface, station 24 thus
reduces the likelihood that a released particle will settle back
down on another part of the mask surface. Rapid and efficient
removal of the released particles is very important, because when
released particles do settle back down on the mask, they may be
even harder to remove than they were initially.
[0049] Processor 37, in addition to receiving particle coordinates
from the inspection station, preferably controls and coordinates
other aspects of station 24, including the laser beam, the process
gas and exhaust flows, and the chuck.
[0050] Although IR laser radiation has typically been used in the
past to cause evaporative explosion of water film on semiconductor
wafer cleaning (as described in the previously-mentioned U.S. Pat.
No. 4,987,286), this approach may not be appropriate for advanced
masks such as an Extreme Ultra Violet (EUV) mask. Such masks
typically comprise 40 or more alternating layers of Si and Mo. The
Si layers are practically transparent to IR radiation, while the Mo
layers strongly absorb IR radiation. Therefore, IR irradiation of
such a mask may cause differential heating of the Mo layers,
leading to undesirable strain in the multilayer structure.
[0051] Therefore, to avoid undesirable thermal effects and possible
substrate damage, the laser beam preferably comprises visible or
ultraviolet (UV) radiation, at wavelengths selected to accommodate
the absorption characteristics of Mo and Si. Examples of two
specific wavelengths that may be used for this purpose are 248 nm
and 532 nm. In addition, IR radiation may be used, preferably at
2940 nm (tuned to the O--H stretch mode of water, for maximum
radiation absorption in the liquid film on the mask surface).
Experimental cleaning results using these wavelengths are described
below.
[0052] FIG. 3 is a schematic top view of a blank mask 200 on which
known particles have been deposited. Mask 200 comprises 40
alternating layers of Si and Mo with thicknesses of 40 .ANG. each
on a Si substrate. Groups of calibrated particles are deposited on
mask 200 as follows: group 210--Si 0.5 .mu.m; group
220--Al.sub.2O.sub.30.4 .mu.m; group 230--SiO.sub.2O 0.4 .mu.m; and
group 240--Al 0.5 .mu.m. Station 24 was applied to remove the
particles from the mask, using laser radiation at the three
wavelengths previously noted (532 nm, 248 nm, and 2940 nm).
[0053] FIG. 4 is a schematic top view of the mask of FIG. 3 showing
cleaning results obtained using laser radiation at 532 nm, 248 nm
and 2940 nm. Arrows 310 indicate cleaning passes performed on
groups 210, 220, 230 and 240 using laser radiation at 532 nm and at
248 nm. Arrows 315 indicate cleaning passes performed at 532 nm.
Arrows 320 indicate cleaning passes performed at 248 nm. Arrows 330
indicate cleaning passes performed at 2940 nm. Each of the three
radiation wavelengths yielded good cleaning results.
[0054] In all cases, damage to the blank mask was rarely observed.
Any damage observed was limited to parts of the mask where
particles were located. Damage could be attributable to lack of
synchronization between the laser beam firing and the flow of gases
yielding the water film on the surface. Another possible
contributor to damage was poor laser beam quality, as a correlation
was found between damage locations and hot spots in the laser beam
profile. Damage was observable in the form of mask reflectivity
changes. Such changes, when they occurred, could be observed with a
naked eye under strong light illumination and appeared to be
similar in shape to the laser beam spot profile. These results were
also observed using instruments including a Tencor 7200 wafer
scanner, an optical microscope, scattered probe laser beam images,
and a SEM. Surprisingly, little or no mask damage was observed at
the 2940 nm laser wavelength. This result may have been due to
increased IR absorption in the thin Si layers because of increased
free electron density, which would tend to balance out the higher
absorption in the Mo layers.
[0055] The examples shown in FIGS. 3 and 4 above are for
illustrative purposes only and are not intended to limit the method
of localized cleaning to a given type of laser, laser wavelength,
or specific mask type. In general, other types of lasers and laser
wavelengths may be used, according to specific needs.
Alternatively, particle removal station 24 may use other local
cleaning methods in conjunction with inspection station 22. For
example, station 24 may apply localized plasma, pressurized gas or
vacuum, or carbon dioxide "snow" (dry ice--using a special nozzle
such as those produced by Applied Surface Technologies). In
addition, station 24 may dispense chemical cleaners in liquid
and/or vapor state locally at the particle coordinates.
[0056] FIG. 5 is a simplified pictorial illustration of a particle
removal station 350, in accordance with an alternative embodiment
of the present invention. The principles of operation of station
350 are described in detail in U.S. patent application titled
"CONDENSATION-BASED ENHANCEMENT OF PARTICLE REMOVAL BY SUCTION",
application Ser. No. 10/035,972, filed Sep. 11, 2001, which is
assigned to the assignee of the present patent application, and
whose disclosure is incorporated herein by reference.
[0057] Station 350 comprises a fluid delivery unit 370 and a
suction unit 380. The fluid delivery unit deposits a fluid,
preferably a vapor, onto mask 26 at the locations of contaminants
determined by inspection station 22, and the suction unit then
removes the contaminants together with the fluid. The introduction
of the fluid onto the particle, coupled with a turbulent
mass-transfer regime surrounding the particle induced by a suction
force from suction unit 380, introduces a mechanical shock to the
particle. The mechanical shock, coupled with the dissolution forces
of the particle into the fluid phase, tends to release the particle
from the surface of the mask. Alternatively, the local suction unit
may be used alone, without wetting the mask.
[0058] Fluid delivery unit 370 comprises a gas inlet valve 362, and
a gas-conveying channel 364. This channel conveys nitrogen or an
inert gas to a vaporizer chamber 368. The chamber is normally
constructed with an external heating jacket 366 and a liquid entry
channel (not shown). The liquid may be, for example, water, a
solvent, or an aqueous solution. The liquid is heated, typically
from 40-80.degree. C., by jacket 366 so as to be partially or fully
vaporized or to enter a gaseous phase. This phase or combination of
liquid and/or vapor and/or gaseous phases is defined herein broadly
as a fluid. The fluid may thus also comprise steam.
[0059] The fluid is conveyed from vaporizer 368 via a fluid channel
372 to the surface of mask 26. Channel 372 is typically heated
externally by a heating jacket 374 or other means known in the art.
Typically, vaporizer 368 comprises a heating element 365, which is
configured to transfer heat to heating jackets 366 and 374.
Preferably, heating jacket 374 is extended so as to heat a fluid
delivery channel 384 and a suction channel 382 concomitantly.
[0060] Suction unit 380 typically comprises a vacuum or
displacement pump (not shown) which introduces reduced pressure or
vacuum forces to a channel 386. The channel may be under continuous
or non-continuous suction. The suction is controlled by the
activation of a valve 388 leading to channel 386. Channel 386 leads
to a nozzle assembly 385 having two channels. A fluid delivery
channel 384, typically the inner channel, conveys the fluid phase
to mask 26. A suction channel 382, typically an outer annular
channel, conveys particles and fluid from the surface under suction
forces.
[0061] Mask 26 is typically supported on an x-y stage 352 with the
fluid delivery and suction channels 384 and 382 controlled to reach
any coordinate on the x-y stage. Alternately, the stage may be a
rotating stage, and the channels may be operated to reach any point
by radial movement.
[0062] FIG. 6 is a simplified block diagram showing a lithography
tool 400 with integrated local particle removal, in accordance with
an embodiment of the present invention. All the elements of tool
400, including inspection station 22 and particle removal station
24 (as shown initially in FIG. 1), are maintained in a controlled
environment within an enclosure 405. Thus, masks generally need not
be removed from enclosure 405 for particle removal, and exposure of
the masks to environmental contaminants is accordingly reduced. The
entire interior of enclosure 405 may be evacuated if desired.
[0063] Lithographic masks to be used in tool 400 are inserted into
a mask storage station 410 through an exterior port 415. The masks
held in station 410 are preferably inspected and cleaned before
use, using the inspection station and particle removal station.
Preferably, masks in storage are inspected and cleaned periodically
even when not in use, as well, in order to promptly remove any
particles that may have adhered to the mask. Masks are transferred
from the mask storage station to the inspection station by a robot
425, without human contact. Inspection station 22 inspects the
surface of the mask and detects any contaminant particles that may
be present on its surface. If the mask is found to be clean of
particles, robot 425 transfers the mask to an exposure station 430
or 432, which exposes the lithographic pattern of the mask onto at
least one semiconductor wafer. Alternatively, if the mask is not
required for use in one of the exposure tools, it is returned to
storage station 410. Mask transfers between the exposure tools
and/or between the mask storage station are effected by the
robot.
[0064] After each use of the mask in exposure tool 430 or 432, the
mask is preferably re-inspected by the inspection station. In this
way, it is ensured that any particles that may be deposited on the
mask are detected and subsequently removed before the next time the
mask is used. Alternatively, for increased throughput of tool 400,
the masks are re-inspected only after having been used for a
certain number of exposures.
[0065] If the inspection station determines that particles must be
removed from the mask, robot 425 transfers the mask to particle
removal station 24 and particles are removed as described
previously with reference to FIG. 2. When removal of particles from
the mask is completed, robot 425 conveys the mask back to
inspection station 22 for re-inspection. If contaminants are still
found on the mask, another round of particle removal may be
performed by particle removal station 24. If the mask cannot be
satisfactorily cleaned even after repeat treatment, robot 425
preferably transfers the mask back to storage station 410 from
removal from enclosure 405. Otherwise, the clean mask is passed to
exposure tool 430 or 432, or it is returned to mask storage station
410 for future use.
[0066] It should be noted that although two exposure stations 430
and 432 are shown in FIG. 5, tool 400 may comprise a larger or
smaller number of exposure stations. The controlled environment of
enclosure 405 ensures that masks are exposed to a minimal number of
particulate contaminants, by restricting human contact with the
masks and by high air filtration or evacuation of the enclosure.
The only regular access to the interior of tool 400 is through port
415 of mask storage station 410, as well as through a similar port
(not shown in the figure) for moving process wafers into and out of
exposure tools located within enclosure 405.
[0067] It will be appreciated that the embodiments described above
are cited by way of example, and that the present invention is not
limited to what has been particularly shown and described
hereinabove. Rather, the scope of the present invention includes
both combinations and subcombinations of the various features
described hereinabove, as well as variations and modifications
thereof which would occur to persons skilled in the art upon
reading the foregoing description and which are not disclosed in
the prior art.
* * * * *